This sonnet belongs to a collection of 154 sonnets.
The tytle hints nothing, because it consists of a number.

Considering the layout the reader understands that it is a Shakespearean sonnet, because it is composed of fourteen lines of iambic pentameter.
It is made up of three quatrains and one concluding couplet.

The function of the first quatrain is to introduct the theme by using a metaphore. Shakespeare
compares his age to a late autumn in which the leaves fall from the threes.

The second quatrain consists of a metaphore in which the age of the poet is compared to a  like late twilight when light is replaced by darkness.

In the third quatrain the speaking voice compares himself to the ember of a fire, which lies on the ashes of the wood that once enabled it to burn.

In the last couplet the speaking voice tells the reader that his love must be strengthened by the awareness that they will be soon apart separated.

Shakespeare's use of language is rich of metaphors and similies referred to the poet's age, which gives the idea of his point of view about his age.
The theme of the poem is "mortality" and the shift from the end of the youth and the approach to death.